Combustion of concentrates resulting from ultrafiltration of bleached-kraft effluents

This paper reports that closed cycle concentrates were simulated by laboratory-scale ultrafiltration of (C + D) and first E-stage effluents from two bleached-kraft mills. The concentrates were found to be similar to typical kraft black liquor in carbon content, heating value, and density, Viscosities of concentrates were similar to or slightly above the values over which black liquor viscosities commonly range. Burning rates for the dried solids from the concentrates were found to be similar to burning rates reported for black liquor. Volatiles yields (percentage of initial dried solids evolving as pyrolysis gases under pyrolytic conditions) were 46-50% and were not strongly dependent on final heating temperature. Char yields (percentage of initial dried solids gasified by heterogeneous char oxidation reactions) were strongly dependent on final heating temperature, increasing from nearly zero at 600{degrees} C to 42% at 1000{degrees} C. A simplistic combustion rate model was developed to predict the overall burning rates. It should prove useful for approximating the rates of combustion of ultrafiltration concentrates. The model includes a first-order rate expression for volatiles evolution and a pseudo-first order rate expression for char burning.